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Routing Protocols

Routing Protocols. RIP and OSPF. Introduction - IP Protocol. Classful IP Addresses. IP address IPv4: 32-bit address dotted-quad or dotted decimal ex. 130.221.203.154 decimal = 82.DD.CB.9A hex = 1000 0010 . 1101 1101 . 1100 1011 . 1001 1010 binary

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Routing Protocols

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  1. Routing Protocols RIP and OSPF

  2. Introduction - IP Protocol

  3. Classful IP Addresses • IP address • IPv4: 32-bit address • dotted-quad or dotted decimal • ex. 130.221.203.154 decimal = 82.DD.CB.9A hex • = 1000 0010 . 1101 1101 . 1100 1011 . 1001 1010 binary • only 232 (4,294,967,296) IPv4 addresses available • 2 parts: netid & hostid • “Classful” addressing • Class A • Class B • Class C • Class D - Mulitcast • Class E - Reserved for future use

  4. Classful IP Addresses - Class A • starts with binary 0 • 27 - 2 (126) Class A networks • 2 reserved Class A networks • 00000000 ( 0.0.0.0 ) : default route • 01111111 ( 127.0.0.0 ) : loopback • 224 - 2 (16,777,214) hosts per Class A network • all 0’s : ‘this network’ • all 1’s : ‘broadcast’ Lowest network address : 1.0.0.0 Highest network address: 126.0.0.0 0 1 2 3 4 8 16 24 31 0 netid hostid 7 bits 24 bits

  5. Classful IP Addresses - Class B • Start with 10 • Second Octet also included in network address • 214 = 16,384 class B addresses Lowest address : 128.0.0.0 Highest address: 191.255.0.0 0 1 2 3 4 8 15 16 24 31 netid 1 0 hostid 14 bits 16 bits

  6. Classful IP Addresses - Class C • Start with 110 • Second and third octet also part of network address • 221 = 2,097,152 addresses Lowest address : 192.0.0.0 Highest address: 223.255.255.0 0 1 2 3 7 15 23 24 31 hostid 1 1 0 netid 21 bits 8 bits

  7. Classful IP Addresses - Class D • Start with 1110 • IP Multicasting Lowest address : 224.0.0.0 Highest address: 239.255.255.255 0 1 2 3 7 15 23 24 31 1 1 1 0 28 bits

  8. Classful IP Addresses - Class E • Start with 1111 • Experimental Lowest address : 240.0.0.0 Highest address: 255.255.255.254 0 1 2 3 7 15 23 24 31 1 1 1 1 28 bits

  9. Classless Interdomain Routing (CIDR) • pronounced “cider” • RFC 1518 & 1519 • addresses two scaling problems on the Internet • growth of backbone routing tables • potential for the 32-bit IP address space to be exhausted • Current IP address inefficiency exists because of the address Class requirements (i.e., A, B, C, etc.) • a network with 2 hosts needs a Class C network address space (2/254 = 0.79%) • for 256 hosts ->Class B (256/65,534 = 0.39%) • Class B exhaustion is more severe - so give out multiple Class C’s • If one AS has 16 Class C’s, each backbone router would need 16 routing table entries for that one AS

  10. CIDR • CIDR helps to aggregate routes • hand out contiguous blocks of Class C addresses • ex: 192.4.16 - 192.4.31 • 16 Class C’s • They all start with 1100 0000 - 0000 0100 - 0001 . . . • looks like a 20-bit network number - something between a Class B and a Class C • each block must contain a number of Class C networks that is a power of 2 • need a routing protocol that can deal with these “classless” addresses (i.e., a non-standard network number) • BGP version 4 is able to do this • Network numbers are represented by (length, value) pairs • example above would have length = 20 • similar to the (mask, value) for subnets

  11. IP Routing • In a packet-switched network, routing relates to the process of choosing a link to send the packets over. • Router: the computer that makes this choice. • an internet is composed of multiple physical networks interconnected by computers (or network devices) called routers • forwarding:take a packet, look at its destination address, consult a table, send packet to its destination based on that table • routing: process which builds the forwarding tables

  12. IP Routing • For routing to scale, a hierarchical routing infrastructure is used (Internet) • Autonomous System (AS) • a group of routers exchanging info with a common routing protocol • a set of routers and networks managed by a single organization • connected (except during failures): a path exists between any two pair of nodes • Interior Gateway Protocols (IGPs) - within an AS • Exterior Gateway Protocols (EGPs) - between ASs

  13. IP Routing IGP1 BGP IGP2 AS #2 AS #1 Interior Gateway Protocol (IGP) Exterior Gateway Protocol (EGP)

  14. IP Routing • routing: graph theory problem • nodes : hosts, switches, routers, or networks • initial case: consider all nodes as routers • edges of graph: network links (assume undirected) • cost is associated with each edge • relates to desirability of sending traffic over particular link • routing problem: find the lowest-cost path between any two nodes • cost = sum of costs of all of the edges that form the path edge (link) 6 3 A node (router) 4 B F E 1 C 2 D 9 1 cost

  15. IP routing • for a simple network • calculate all shortest paths and store in a table • problems with this static approach: • does not handle node or link failures • does not consider addition of new nodes or links • assumes fixed edge costs (may want to adjust cost upward for increased loading) • to deal with the static routing problems, routing protocols are used between nodes to discover the lowest cost paths and are • distributed • centralization inhibits scalability • distributed algorithms may cause synchronization problems • dynamic

  16. IP intradomain routing • Look at two main classes of IGPs • distance vector (RIP) • link state (OSPF) • assume all edge costs are known

  17. Routing Information Protocol (RIP) • built on a Distance-Vector algorithm • also called Bellman-Ford algorithm, after the inventors • each node constructs a one-dimensional array (a vector) of the “distances” (costs) to all of the other nodes and distributes the vector to its immediate neighbors • assumes that each node knows the cost of the links to its immediate (directly connected) neighbors • a link outage is assigned an infinite cost • assume each link cost = 1, so now least-cost path is the fewest number of router hops • One of the most widely-used routing protocols • RFC 1058

  18. RIP example Info Stored Distance to reach node at node A B C D E F G A 0 1 1  1 1  B 1 0 1     C 1 1 0 1    D   1 0   1 E 1    0   F 1     0 1 G    1  1 0 Destination Cost NextHop B 1 B C 1 C D  - E 1 E F 1 F G  - initial distances/costs stored at each node initial routing table at node A

  19. RIP example, cont. Info Stored Distance to reach node at node A B C D E F G A 0 1 1 2 1 1 2 B 1 0 1 2 2 2 3 C 1 1 0 1 2 2 2 D 2 2 1 0 3 2 1 E 1 2 2 3 0 2 3 F 1 2 2 2 2 0 1 G 2 3 2 1 3 1 0 Destination Cost NextHop B 1 B C 1 C D 2 C E 1 E F 1 F G 2 F final distances/costs stored at each node final routing table at node A

  20. RIP example, cont • in the absence of any topology changes, only a few exchanges are required between neighbors before each node has completed its routing table • convergence: the process of getting consistent routing information to all of the nodes • there is no one node in the network that has all of the information in the complete routing table • each node only has knowledge of its own routing table • each node has a consistent view of the network in the absence of any centralized authority • routing updates • periodic (seconds to minutes) • triggered

  21. RIP, example • RIP packets advertise costs to reach networks (rather than routers/ nodes) command: ‘1’ (request), ‘2’ (reply) version: ‘1’ (or ‘2’ for RIPv2) address family: ‘2’ (IP) address: IP address distance: cost metric - hop count up to 25 routes per RIP message well-known RIP port: UDP 520 RIP packet format

  22. RIP • RIP messages carried in UDP datagrams • RIP version 2 (RFC 1388) • RIP-2 • pass additional information • routing domain, route tag, subnet mask • interoperable with RIP • cisco’s proprietary distance-vector Interior Gateway Routing Protocol (IGRP)

  23. Open Shortest Path First (OSPF) • another intradomain or interior gateway protocol (IGP) • Link-state • ‘Open’ : non-proprietary (IETF) vs. proprietary EIGRP (cisco) • RFC 1247 • each node is assumed to be capable of finding the state of the link to its neighbors (up or down) and the cost of each link • assume • reliable dissemination of link-state info • reliable flooding (all of node’s L-S info to all attached nodes) • update packet (link-state packet [LSP]) • calculation of routes from the sum of all the accumulated link-state knowledge • Dijkstra’s shortest-path algorithm

  24. OSPF • Uses IP directly (does not use UDP or TCP) • has it’s own value (protocol ID) in the IP header • can calculate a separate set of routes for each IP type-of-service • there can be multiple routing table entries for any destination, one for each TOS • each interface is assigned a dimensionless cost • can be throughput, RTT, reliability, etc. • separate cost for each TOS

  25. OSPF • when more than one equal-cost routes exist to a destination, OSPF distributes traffic equally among routes (load balancing) • supports subnets: an associated subnet mask with each advertised route • allows a single IP address of any class to be broken into multiple subnets of various sizes (variable-length subnets) • simple authentication scheme (cleartext password, similar to RIP-2) can be used • replaces RIP

  26. Exterior Gateway Protocols (EGPs) • interdomain routing protocols • used between routers of different AS’s • historically, the predominant EGP was a protocol called EGP (confusing) • the newer EGP is the Border Gateway Protocol (BGP) • version 3 (RFC 1267) • RFC 1268 (use of BGP in the Internet) • version 4 (RFC 1654) • message types (RFC 1771) • updates sent using TCP

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